Lactide is a well-known intermediate product in the manufacturing of polymer materials, like polylactic acid (PLA) or PLA-containing copolymers. Lactide (sometimes called dilactide) is a cyclic dimer of lactic acid and is usually manufactured by means of a two-step process. In the first step, lactic acid is polymerized into a so-called pre-polymer or oligomer, having a relatively low molecular weight. In the second step, crude lactide is formed from this pre-polymer by means of a so-called ‘backbiting’ process in the presence of a catalyst. This crude lactide material may be purified by means of (repeated) crystallization and/or (repeated) distillation. The so-obtained purified lactide may subsequently be used in a polymerization process for the manufacture PLA or PLA-containing copolymers.
It is well-known that lactide can exist in three different geometric structures, which have a diastereomeric relationship. These different structures can be distinguished as R,R-lactide (or D-lactide), S,S-lactide (or L-lactide) and R,S-lactide (or meso-lactide). Mixtures of equal amounts of D- and L-lactide are often referred to as racemic lactide or rac-lactide. Within the scope of the present invention, the word ‘lactide’ refers both to the three pure lactides (being composed of only one diastereomer) as well as to mixtures of two or more of the pure lactides.
The purity of lactide is an important issue. This is especially true as impurities may have a strong influence on the polymerization of lactide into PLA. In view therefore, it is relevant to have available methods which allow the determination of the amounts of impurities in lactide. Such methods should have a high accuracy and reliability. Such methods should moreover be simple in their use and implementation in lactide handling processes.
Well-known impurities in lactides are species with hydroxyl groups and/or carboxylic acid groups. Water and free acid species are important examples of these types of impurities. In view thereof, the amount of such impurities in the lactide material should be kept as low as possible. Repeated distillation and repeated crystallization techniques of different types are well-known technical purification means which can be used during the production of lactide for lowering the amount of the mentioned impurities as much as possible.
Currently, titration methods are often used to determine the amount of impurities, like species with hydroxyl groups and/or carboxylic acid groups, in lactide. For executing these methods, small samples of lactide-containing material need to be taken and handled by different titration procedures in order to determine the exact amount of different impurities.
According to the experience of the inventors, the known titration methods for quantification of impurities in lactide are rather cumbersome and labor-intensive in their use. Moreover, the results of such quantification methods are not immediately available. Therefore, determination of the impurities by means of the known titration methods has the drawbacks of being relatively expensive and less suitable to monitor the lactide quality online under mass production circumstances.